This invention relates to vapor phase growing apparatus for growing P or N type semiconductor layers on semiconductor wafers or substrates.
In semiconductor vapor phase growing apparatus, semiconductor substrates, for example, monocrystalline silicon wafers are placed in a gaseous mixture of hydrogen gas acting as a reaction gas, silicon tetrachloride acting as a carrier gas and a dopant gas acting as an impurity, and then heated to a temperature of about 1200.degree. C. to cause a chemical reaction so as to form silicon layers on the wafers.
Thus, H.sub.2 and SiCl.sub.4 are caused to react with each other as follows. EQU 2H.sub.2 +SiCl.sub.4 .revreaction.Si+4HCl
Since the gaseous mixture contains a dopant gas the layers formed on the wafers are N type or P type silicon layers.
Since the wafer comprises a monocrystalline silicon substrate, its electroconductivity is extremely low so that it is possible to alternately deposit N type and P type silicon layers and to form an IC chip by forming a pattern of an integrated circuit.
A typical example of semiconductor vapor phase growing apparatus is shown in FIG. 1 comprising a source of power, usually a high frequency generator, reaction furnaces R1 and R2 for effecting vapor phase growth, operating panels 12A and 13A provided for the main bodies 12 and 13 of the reaction furnaces R1 and R2 for opening and closing thereof, control unit 14 for controlling flow quantities of gases supplied to the reaction furnaces R1 and R2 and temperatures thereof, and an operating panel 14A of the control unit 14.
As shown in FIG. 2, each reaction furnace comprises a bottom plate 21 and a dome or lid 22 covering the bottom plate.
An inlet pipe 23 is provided at the center of the bottom plate 21 to admit reaction gases into the furnace through an inlet port at the top of the pipe 23. A rotary shaft 25 extends through the bottom plate 21 to concentrically surround the pipe 24 for supporting a circular disc shaped susceptor or support 26. The rotary shaft 25 is driven by an electric motor 27 having a reduction gearing.
Beneath the support 26 is disposed an induction heating coil 29 covered by a cover 28. The coil 29 is supported by an insulating plate 30 secured to the bottom plate 21 through bolts 31. Terminals 32 and 33 connected to the induction heating coil 29 extend through the bottom plate 21. Cooling water is passed through the induction heating coil 29 for removing heat generated by high frequency current flowing through the coil 29.
The dome 22 is of a three layer construction comprising an inner quartz layer 34, a first stainless steel layer 35, and a second stainless steel layer 36 which are spaced by air gaps. An observation window 38 extending through the first and second stainless steel layers 35 and 36 is provided for the top portion of the dome to observe the wafers 37 mounted on the rotary support 26 and a temperature detection window 39 provided with a temperature sensor TS for measuring the temperature of the furnace is also mounted on the top of the dome.
A clamping member 40 operated by an air cylinder device 41 is provided for cooperating with a flange 42 of the dome 22 to mount and dismount the same.
A bracket 43 is integrally formed with the dome 22 and moved in the vertical direction by a cylinder, not shown, to raise the dome 22 for exchanging the wafers 37 on the support 26. An exhaust pipe 44 is connected to the bottom plate 21.
To exchange wafers in the reaction furnaces R1 and R2, the operating panels 12A and 13A are operated to open the domes 22. Then wafers 37 on the support 26 are exchanged with new ones and then the domes are closed by manipulating operating panels 12A and 13A.
Under these conditions, the domes 22 are brought into intimate contact with the bottom plates 21 to air-tightly seal the reaction furnaces. After that, the control unit 14 is operated for starting the vapor phase growth. Firstly, the control unit 14 supplies gases into the furnaces and controls the high frequency generator 11 to adjust the current value supplied to the induction heating coils.
Accordingly, gases are supplied into the reaction furnaces through pipes 24 and high frequency magnetic fields are produced corresponding to the current values, thus heating the wafers on the supports 26 by induction. At the same time, the temperature in the furnaces rises so that the gases supplied into the furnaces undergo above described chemical reaction to grow P type or N type silicon layers on the wafers 37.
As a result of the chemical reaction of the gases, the compositions of the gases change so that it is necessary to constantly exhaust gases in the furnaces to always fill the furnaces with fresh gases. The supports 26 are rotated by motors 27 to subject wafers 37 to the same condition at any position, thus making the thickness of the silicon, films grown by vapor phase method the same for all wafers.
Since the reaction temperature in the furnaces is about 1200.degree. C., at start the furnace temperature is raised to 1200.degree. C. from room temperature. However, when the temperature is raised rapidly the wafers crack resulting in a so-called slip phenomenon. Accordingly, it is necessary to control the output V of the high frequency source 11 such that the temperature of the furnaces vary linearly, as shown in FIG. 3.
Such control is made with the control unit 14. But in the past, a sequence controller has been used as the control unit 14 to operate as follows.
As is well known in the art, the sequence controller performs predetermined controls for predetermined intervals according to a predetermined order, and is constructed as shown in FIG. 4 for example.
In FIG. 4, A designates a pin board switch panel arranged in a matrix comprising a plurality of bus lines spaced in the directions of column and row. At crosspoints of the bus lines, openings for receiving pins for short circuiting are provided. In FIG. 4, the order of the sequences is shown in the direction of the column, and the contents (the type of the process sequence) to be executed are shown in the direction of the row.
Thus, the order of executions of the sequences is shown by steps 1 through 7 while the type of the process sequences is shown by PP1 through PP17. The sequence controller is constructed to designate the execution times of respective steps 1 through 7. Thus, the types of the sequences to be executed at steps 1-7 are selected among PP1-PP17 and pins are inserted at positions of the selected types. In FIG. 4, dark spots show the positions at which pins are inserted.
B designates a relay ladder circuit incorporated with relay circuit enabling controls corresponding to the types of the sequences. Accordingly, the relay ladder circuit B produces a control output corresponding to the type of sequence instructed by the pin board switch panel A.
When the pins are set as shown in FIG. 4, the control unit 14 searches for what type of sequence has been set at step 1. In other words, a bus line in the direction of a row connected by a pin to a bus line corresponding to the step 1 is searched. Since at step 1, PP2 is connected by a pin, the bus line of PP2 would be detected when it is searched.
The detected information is sent to the relay ladder circuit B which operates the relay circuit for effecting a control corresponding to PP2 according to the information thereof. At this time, since a time information for executing the control is simultaneously given from the pin board switch panel A a control is executed for an interval corresponding to the time information. As this interval elapses, the same operation is carried out for the next step 2 to give an information PP3, thus causing the relay circuit to execute a control corresponding to PP3. Accordingly, process sequences are executed in an order of PP2.fwdarw.PP3.fwdarw.PP1 .fwdarw.PP4.fwdarw.PP6.fwdarw.PP5.fwdarw.PP7.
However, the control device described above merely applies control signals for controlling valves of various gases and cooling water, and for supplying and interrupting heating power.
In contrast, the flow quantities of the gases and the furnace temperature, the most important factors in actual operation, are set by such setters as variable resistors. In other words, these important factors are not controlled by the control device.
For example, an inclination angle .theta. (FIG. 3) of the source output is set by a setter so as to gradually increase the source output as shown in FIG. 3, while only the heating time is controlled by the sequence controller.
However, the temperature of the support 36 adapted to support wafers 37 rises slowly as shown in FIG. 5 due to the heat capacity of the support thus failing to follow up the increase in the source output. More particularly, the temperature of the support increases very slowly at the start and rises abruptly at an intermediate point. Accordingly, the wafers 37 are subjected to this abrupt temperature variation, thus causing slip.
These problems have been solved by thickening the support or by gradually increasing the furnace temperature. However, these measures prolong the vapor phase growing time, thus increasing the cost of the products.
Another method of solving these problems utilizes a commercial temperature controller. This method can eliminate the defects of the first method in which the inclination angle .theta. is set by a variable resistor.
More particularly, as shown in FIG. 6 the heating time required for the support to reach the operating temperature 1200.degree. C., is divided into a plurality of sections P.sub.1, P.sub.2, . . . P.sub.n by taking into consideration the temperature characteristic of the support, and rates of temperature variations of respective sections are preset so that the output increase rate of the source 11 will become high in time zones in which the rate of temperature rise of the support is low, whereas the output increase rate will be low in time zones in which the rate of temperature rise of the support is high.
Although this method enables to linearly control the temperature of the support it is necessary to set the rate of change at many points and to measure the actual temperature characteristic of the support for effecting the temperature change rates. Such troublesome measurement and setting must be made each time the support is exchanged.
Where the temperature is repeatedly raised and lowered in one process sequence and where the selected inclination angle .theta. differs in respective sequences, it is necessary to provide the temperature controllers of the same number as that of the sequences, thus increasing the cost of the control system.